Microstructured optical fiber waveguides are well known in the prior art. Of all such microstructured fibers, photonic bandgap fibers are among the most important due to their ability to transmit optical signals with very low losses and with very low non-linearity. Unlike conventional optical fibers having a core formed from a higher index glass that is surrounded by a cladding of lower index glass, and which rely exclusively on total internal reflection (TIR) to conduct a beam of light, microstructured optical fibers typically have a hollow core surrounded by a pattern of alternating low and high index materials that conducts light by imposing a “forbidden zone” on a range of optical wavelengths that cannot propagate through the microstructure surrounding the hollow core. The pattern of alternating low and high index materials may take the form of a pattern of holes in the glass surrounding the core, but may also be (in the case of Bragg type fibers) alternating concentric rings of high and low index materials that surround the core. When a beam of light is introduced into the hollow core of a fiber having such a microstructure, light having a wavelength within the “forbidden zone” is trapped within the core and conducted through the fiber, while light having a wavelength outside the “forbidden zone” propagates through the microstructure and out of the sides of the fiber. Because the optical signal conducted through the hollow core of a photonic bandgap fiber (which is typically air filled) incurs much less absorption and Rayleigh scattering and practically none of the nonlinearities imposed by the glass cores of more conventional fibers, there is currently a great deal of interest in the economical manufacture of such fibers.
In addition to photonic bandgap fibers, there is interest in other types of microstructured fiber waveguides as well. For example, microstructured optical fibers having a solid glass core and a plurality of holes disposed in the cladding region around the core have been constructed. The arrangement, spacings and sizes of the holes may be designed to yield microstructured optical fibers with dispersions ranging anywhere from large negative values to large positive values. Such fibers may be useful, for example, in dispersion compensation. Solid-core microstructured optical fibers may also be designed to be single mode over a wide range of wavelengths. Solid-core microstructured optical fibers generally guide light by a total internal reflection mechanism; the low index of the holes can be thought of as lowering the effective index of the cladding region in which they are disposed.
Microstructured optical fibers are fabricated using methods roughly analogous to the manufacture of all-glass optical fiber. A structured preform having the desired arrangement of holes is formed, and then drawn into fiber using heat and tension. In the stack and draw method of making a microstructured optical fiber preform, hollow glass tubes which are typically hexagonal-sided are stacked together to form an assembly having the desired lattice structure, and one or more of the tubes are removed to form a core volume in the center of the lattice structure. The lattice structure is fused and redrawn to reduce its cross-sectional size, then sleeved and drawn into optical fiber having a lattice-like array of holes surrounding a core defect void. Preforms made according to the stack and draw process are categorized as either close packed arrays or non-close packed arrays. A close-packed array is an array of capillary tubes where the capillary tubes are mutually contiguous. A non-close packed array is an array of capillary tubes where jigs or spacers are placed between the capillary tubes to space the walls of the tubes apart a desired distance.
Unfortunately, there are a number of undesirable limitations associated with such conventional methods of forming a microstructure. For example, because the core volume is formed by the removal of one or more tubes from the assembly, it has a shape and size restricted to integer multiples of the shape of the hexagonal-sided tubes of the assembly. Additionally, every particular microstructure pattern (i.e. those having different void spaces and different pitches of air holes, those having multiple hollow cores, and those having solid cores of different pitches and patterns) requires a custom-made preform that in turn requires a particular custom-stacking of hollow glass tubes. In some instances, the desired microstructure cannot be obtained by a simple, close-packed array of identically-dimensioned capillary tubes having hexagonal outer walls that interfit and mutually support one another. Instead, a non-close-packed array of capillary tubes is required wherein specially sized spacers or jigs are placed in the array to space the walls of the capillary tubes apart a desired distance. Finally, there are a number of potentially useful microstructure designs that are difficult if not impossible to produce by such conventional methods, such as a microstructure wherein every other hole has a different diameter. While etching techniques are known which are capable of creating microstructures having different void-filling fractions in a same type of preform, such techniques only partially overcome the aforementioned limitations as the etchant must be conducted through all the holes in the preform simultaneously, thereby increasing the diameter of all the holes the same amount.
Clearly, there is a need for a new manufacturing method that does not require the custom manufacture of a particular preform for every different microstructure design. Ideally, such a method would allow a broad range of different microstructure patterns to be manufactured from a single, standard easily manufactured preform in order to expedite the manufacturing process. Finally, it would be desirable if such a method allowed the manufacture of microstructure patterns that were either impractical or impossible to manufacture by conventional manufacturing techniques.